Electron discharge tube arrangements



July 31, 1956 R. s. WEBLEY ELECTRON DISCHARGE TUBE ARRANGEMENTS Filed Sept. 29, 1951 FIG W fi M /W S H/ W 2 W/ my HALF SlLVEfiED MHZROR Lucsu-r 6 WT cANmuG Toae SW] TCH VOL TAGE SOURCE INSULATING PARTI 01.55

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ELECTRON DISCHARGE TUBE ARRANGEMENTS Reginald Sidney Webley, Hayes, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application September 29, 1951, Serial No. 248,987

12 Claims. (Cl. 1786.8)

This invention relates to electron discharge tube arrangements. It is well known to employ a cathode ray tube for providing optical images in response to electrical signals as for example in television receivers. The optical image is formed on the fluorescent screen of the tube and the screen is normally of such nature that the images persist for only a short time. In some cases it may be required to make a detailed examination of the images formed in response to electrical signals, for example where the tube is employed to provide optical images in response to electrical signals reflected from objects as in radar apparatus but in normal tubes the image does not persist sufiiciently long for such an examination to be made.

The object of the invention is to provide an electron discharge tube arrangement which enables optical images to be produced which are of longer persistence as compared with optical images produced by cathode ray tubes in the normal manner.

According to the invention, there is provided an electron discharge tube arrangement comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material formed to provide a multiplicity of spaces through which electrons can pass, an electrically conducting member arranged to receive electrons which pass through said spaces, means for projecting an optical image on said photocathode to release electrons to form a charge image on said insulating material, means for generating a beam of electrons and for scanning said beam over said insulating material, and means for maintaining said conducting member at a potential to collect electrons which pass through said spaces from said scanning beam and to impart a velocity to said beam substantially limited to the lowest ionization potential of residual gases contained in said envelope.

Preferably the said photo-cathode and target are as close as possible to one another in order to improve the definition of the charge image as compared with an arrangement in which a cathode and a target are provided which are relatively Widely spaced apart. A further improvement in definition can be obtained by providing a high axial magnetic field between the photo-cathode and the target and in one form of the invention this may be achieved by providing an electro-magnetic or a permanent magnet having one pole arranged close to said target.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawing in which:

Figure 1 is a diagrammatic view of an arrangement according to the invention, and

Figures 2 and 3 show different constructions respectively of target electrodes for use in the tube shown in Figure 1.

Referring to the drawing the arrangement shown comprises an envelope 1 which is preferably evacuated to a States, Patent a. ,?57,Z33 Patented July 31,- 1956 rip ' pressure of the order of 5.10-6 mms. mercury and within which are disposed a photo-cathode 2, which may consist of photo-electric material disposed on a conducting semi-transparent carrier, and a target consisting of insulating particles 3 arranged in spaced relation on an electrically conducting plate 4. An enlarged view of one form of target electrode is shown in Fig 2, in which said particles consist of aluminium oxide, magnesium oxide or calcium fluoride and said plate 4 is made from aluminium. Where aluminium oxide is employed as said particles, the target may for example be conveniently made by anodising discrete areas of an aluminium plate. In the alternative form of target, as shown in Fig. 3, the target comprises a conducting plate 4 and a grid 4a, covered with insulating material such as any of those referred to above, disposed in contact with said plate 4 or slightly spaced therefrom, the interstices in said grid permitting electrons to pass through the grid to said plate. The photo-cathode and target are arranged close together and may for example be about 1 mm. apart. Reference numeral 5 indicates a magnetic core which is of re-entrant form as shown and which is arranged with one pole closely adjacent the end wall 6 of the envelope 1. Within the re-entrant part of the core 5 is disposed an energising coil 7. A cathode ray tube 8 is disposed with its fluorescent screen facing the photo-cathode 2 and between tube 8 and envelope 1 is disposed a focussing optical system shown as a lens 9, and a half-silvered mirror it} is disposed between tube 8 and optical system 9 with its plane at 45 to the axis of tube 8. A further cathode ray tube 11 which may be a plan position indicator, is disposed so that images formed on the screen thereof in response to electrical signals are reflected by mirror 10 on to the photo-cathode 2 via the optical system 9.

Plate 4 is connected, via a signal resistance 12 and an amplifier 13 to a viewing cathode ray tube 14 and the photo-cathode 2 is connected via a switch 15 to a voltage source 16. In operation, the plate 4 may be made 2 or 3 volts positive with respect to the photo-cathode 2 which may be at zero potential and the latter is then scanned with a flying light spot produced by causing the beam of cathode ray tube 8 to scan the screen of tube 8, said beam being unmodulated. The photo-cathode is thereby caused to emit electrons, which under the action of the axial magnetic field produced by the core 5 and coil 7 proceed to the target. The particles 3 thereby become stabilised at the potential of the photo-cathode, and due to the potential difierence between said particles and the plate 4 the electrons from the photo-cathode, which approach the particles at very low velocities, pass between said particles to the plate 4, whereby a constant signal current is generated across resistance 12 so that the screen of the viewing cathode ray tube 14 shows a scanned patch which is uniformly white or black according to the type of amplifier 13 employed. The photocathode is then reduced in potential by 3 or 4 volts by operation of the switch 15 and the beam of tube 8 is extinguished, and optical images which have been or are being formed on the screen of cathode ray tube 11 under the action of the electrical signals applied to said tube are reflected by mirror 14) via the optical system 9 on to the photo-cathode 2. The photo-cathode is thereby caused to liberate diiierent numbers of electrons from ditferent parts of its surface according to the light incident 'on the said diflerent parts of the surface, and under the image coresponding to the optical image on the screen of tube 11 is formed on the particles. In order to read the charge image, that is to say to derive signals from it, the photo-cathode is restored to its original potential by operation of the switch 15 and is then again scanned with a flying light spot provided by cathode ray tube 8 as above described. The photo-cathode 2 is thereby caused to emit electrons from all parts of its surface, these electrons proceed in a stream towards the target under the action of the magnetic field. When the electrons approach particles 3 which are at maximum negative potentials coresponding to those parts of the photocathode which receive most light during the formation of the charge image, the electrons are repelled and produce no signal. Electrons which approach particles, the potentials of which have not been lowered, pass between these particles due to the electric field between the particles and the plate 4. Of the electrons which approach particles which are at intermediate potentials some electrons are repulsed and others pass between the particles to the plate 4. The electrons which are collected by the late 4 set up signals across the resistance 12 which are fed to amplifier 13 and are reproduced in tube 14 to provide an optical image corresponding to the optical image formed in tube 11.

The photo-cathode may consist of antimony sensitised with caesium, the ionisation potential of which is 3.86 volts. With the voltages applied to the plate 4 and the photo-cathode 2 as described above, the velocities of the electrons which proceed to the plate 4 are not substantially greater than the lowest ionisation potential of residual gases contained in the tube 1. Consequently, substantially no ionisation will occur, with the result that any fading of the charge image set up on the target will be mainly due to electrical leakage. The target can therefore be scanned a plurality of times without appreciable discharge of the charge image with the result that the persistence of the optical image formed in the screen of tube 14 can be effectively appreciably longer than that formed by tube 11. It has been found that where the insulating material of the target is calcium fluoride the charge image persists for many hours without appreciable fading and with persistent reading of the image. It is, however, preferred that the potentials applied to the plate 4- and the photo-cathode 2 should be below said ionisation potential, in which case the maximum potential difference existing between the photo-cathode and the plate t should not exceed 3.86 volts. This is particularly the case during reading of the charge image and is also preferably the case during writing of the charge image. The actual potentials chosen will, of course, depend on the material of which the photocathode is composed, since the ionisation potential differs with different materials. For example, the ionisation potential of a photo-cathode composed of potassium is 4.33 volts, rubidium 4.13 volts and sodium 5.01 volts.

The electrons which pass from the photo-cathode to the target execute minute helices under the action of the axial magnetic field and by virtue of the high magnetic fieid employed and the close disposition of the photocathode and target a high definition can be obtained. For example, in a field of 5,000 oersteds the radius of a helix in which the electron has one electron volt of lateral energy is 0.007 millimeter.

in order to cischarge the charge image formed on the target 4 when desired, the photo-cathode is brought to a relatively high negative potential by operation of the switch 15, for example 100 volts negative with respect to the plate 4 and the photo-cathode is then scanned by the light spot from the tube 8 so that the velocity of the electrons which proceed from the photo-cathode 2 is then such that the target is scanned at a point beyond the first cross-over point of the insulating material of the target electrode, whereby secondary electrons are emitted from the particles 3 so as to cause them to become a few volts positive with respect to the plate 4. The potential of the photo-cathode is then brought to zero volts and again scanned so that the particles 3 then become stabilised at the potential of the photo-cathode as described above.

What I claim is:

1. A circuit arrangement embodying an electron discharge tube comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material providing a multiplicity of spaces through which electrons can pass and an electrically conducting member arranged to receive electrons passing through said spaces, means for projecting an optical image on said photocathode to release electrons to form a charge image on said insulating material, means for generating a beam of electrons and means for scanning said beam over said target electrode, and 'a source of potential for maintaining said conducting member at a potential to collect electrons passing through said spaces from saidv scanning beam and to impart a velocity to said beam which is so low that substantially no ionisation is produced as a result of the interaction between said beam and residual gases in said tube whereby loss of charge due to ionisation is substantially avoided.

2. A circuit arrangement according to claim 1 having means for producing a magnetic field substantially parallel to the axis of said tube and substantially normal to said target and extending from said cathode to said target, said means including a magnet having a pole piece with a face arranged close to and substantially parallel to said target.

3. A circuit arrangement embodying an electron discharge tube comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material providing a multiplicity of spaces through which electrons can pass and an electrically conducting member arranged to receive electrons passing through said spaces, means for projecting an optical image on said photocathode to release electrons to form a negative charge image on said insulating material, a cathode ray tube having a fluorescent screen, means in said cathode ray tube for generating a beam of electrons and means for scanning said beam over said fluorescent screen to generate a flying light spot, means for projecting said flying light spot on to said photocathode to generate a beam of electrons to scan said target electrode, and a source of potential for maintaining said conducting member at a potential to collect electrons passing through said spaces from said scanning beam and to impart a velocity to said beam which is so low that substantially no ionisation is produced as a result of the interaction between said beam and residual gases in said tube whereby loss of charge due to ionisation is substantially avoided.

4. A circuit arrangement according to claim 3 having means for producing a magnetic field substantially parallel to the axis of said electron discharge tube and substantially normal to said target and extending from said cathode to said target, said means including a magnet having a pole piece with a face arranged close to and substantially paral lel to said target.

5. A circuit arrangement embodying an electron discharge tube comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material providing a multiplicity of spaces through which electrons can pass and an electrically conducting member arranged to receive electrons passing through said spaces, means for projecting an optical image on said photocathode to release electrons, a source of potential for maintaining said conducting member at a potential to cause said released electrons to have a velocity which is so low that substantially no ionisation occurs as a result of the interaction between said electrons and residual gases con-- taiued in said envelope and to form a negative charge image on said insulating material, means for generating a beam of electrons and means for scanning said beam over said target electrode, said source of potential being proportioned to maintain said conducting member at a potential to collect electrons passing through said spaces from said scanning beam and to impart a velocity to said beam which is so low that substantially no ionisation is produced as a result of the interaction between said beam and residual gases in said tube whereby loss of charge due to ionisation is substantially avoided.

6. A circuit arrangement according to claim 5 having means for producing a magnetic field substantially parallel to the axis of said tube and substantially normal to said target and extending from said cathode to said target, said means including a magnet having a pole piece with a face arranged close to and substantially parallel to said target.

7. A circuit arrangement embodying an electron discharge tube comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material providing a multiplicity of spaces through which electrons can pass and an electrically conducting member arranged to receive electrons which pass through said spaces, means for projecting an optical image on said photocathode to release electrons to form a charge image on said insulating material, means for generating a beam of electrons and for scanning said beam over said target electrode, and a source of potential for maintaining said conducting member at a potential to collect electrons passing through said spaces from said scanning beam and to impart a velocity to said beam which is so low that substantially no ionisation is produced as a result of the interaction between said beam and residual gases in said tube whereby loss of charge due to ionisation is substantially avoided.

8. A circuit arrangement according to claim 7, having means for producing a magnetic field substantially parallel to the axis of said tube and substantially normal to said target and extending from said cathode to said target, said means including a magnet having a pole piece with a face arranged close to and substantially parallel to said target.

9. A circuit arrangement embodying an electron discharge tube comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material providing a multiplicity of spaces through which electrons can pass and an electrically conducting member arranged to receive electrons passing through said spaces, means for projecting an optical image on said photocathode to release electrons to form a charge image on said insulating material, a cathode ray tube having a fluorescent screen, means in said cathode ray tube for generating a beam of electrons and means for scanning said beam over said fluorescent screen to generate a flying light spot, means for projecting said flying light spot onto said photocathode to generate a beam of electrons to scan said target electrode, and a source of potential for maintaining said conducting member at a potential to collect electrons passing through said spaces from said scanning beam and to impart a velocity to said beam which is so low that substantially no ionisation is produced as a result of the interaction between said beam and residual gases in said tube, whereby loss of charge due to ionisation is substantially avoided.

10. A circuit arrangement according to claim 9 having means for producing a magnetic field substantially parallel to the axis of said electron discharge tube and substantially normal to said target and extending from said photocathode to said target, said means including a magnet having a pole piece with a face arranged close to and substantially parallel to said target.

11. A circuit arrangement embodying an electron discharge tube comprising an evacuated envelope containing a photocathode, a target electrode comprising insulating material providing a multiplicity of spaces through which electrons can pass and an electrically conducting member arranged to receive electrons passing through said spaces, means for projecting an optical image on said photocathode to release electrons, a source of potential for maintaining said conducting member at a potential to cause said released electrons to have a velocity which is so low that substantially no ionisation occurs as a result of the interaction between said electrons and residual gases contained in said envelope and to form a negative charge image on said insulating material, and means for generating a beam of electrons and means for scanning said beam over said target electrode, said source of potential being proportioned to maintain said conducting member at a potential to collect electrons passing through said spaces from said scanning beam and to impart a velocity to said beam which is so low that substantially no ionisation is produced as a result of the interaction between said beam and residual gases in said tube whereby loss of charge due to ionisation is substantially avoided.

12. A circuit arrangement according to claim 11, having means for producing a magnetic field substantially parallel to the axis of said tube and substantially normal to said target and extending from said cathode to said target, said means including a magnet having a pole piece with a face arranged close to and substantially parallel to said target.

References Cited in the file of this patent UNITED STATES PATENTS 2,212,923 Miller Aug. 27, 1940 2,267,823 Goldmark Dec. 30, 1941 2,324,504 Iams et al July 20, 1943 2,411,155 Gorn Nov. 19, 1946 2,495,042 Wilder et al Jan. 17, 1950 2,572,644 McGee Oct. 23, 1951 2,619,531 Weighton Nov. 25, 1952. 2,622,226 Theile Dec. 16, 1952 2,668,190 Sziklai Feb. 2, 1954 

